The subject matter disclosed herein relates generally to converter topologies useful for direct current (DC) power transmission.
In distributed generation applications wherein the site for generation is remote from the available electric grid or load point, bulk power is often transmitted over long distances. In an off-shore wind farm, for example, power generated by individual wind turbine generators is processed by power electronic converters to convert variable voltage, variable frequency output to fixed voltage, fixed frequency output. The outputs from the individual generators are synchronized to the utility network frequency even though the individual machines are running at different speeds and hence outputting different frequencies. The power generated from the turbines is then brought together by a collection system that includes transformers and switchgears for isolating individual turbines and stepping up the voltages, usually to tens of kilovolts. The collection network is then cabled to an off-shore substation that boosts up the voltage further, usually to hundreds of kilovolts. It is then transmitted through subsea cable to an on-shore substation, where it is tied to the utility network through isolating switch-gears and transformers.
For applications wherein bulk power is transmitted over long distances, conventional alternating current (AC) transmission provides technical challenges. Capacitance causes charging current to flow along the length of the AC cable. Because the cable must carry this current as well as the useful source current, this physical limitation reduces the source carrying capability of the cable. Because capacitance is distributed along the entire length of the cable, longer lengths result in higher capacitance and higher resulting charging current. As the cable system design voltage is increased to minimize the line losses and voltage drop, the charging current also increases.
DC transmission can be achieved more efficiently over longer distances than AC transmission. Medium voltage (MV) or high voltage (HV) DC transmission typically requires power electronic converters which are capable of converting between HV AC and HV DC. In conventional converter topologies, each switch of the converter is designed to handle high voltages which may range from tens of kilovolts to hundreds of kilovolts depending upon the application. Such switches are typically arranged with series connection of several semiconductor devices such as insulated gate bipolar transistors (IGBTs) and thyristors.